The invention was designed as an improvement on a commercially available digital radio network of the type known as ADRIS. Such prior art network comprises an array of antennas covering a predetermined geographical area in which many users, each with a hand held or remote terminal (RT), can communicate with a host computer through a plurality of base stations and one or more communications controllers. The antennas are arranged to provide overlapping coverage and include at least one primary antenna providing a primary area of greater coverage and a plurality of secondary antennas having respective secondary coverage areas within the primary area. All radio transmissions between the base stations and the RTs are made at the same carrier frequency so that the system is frequency limited as to the number of transmissions that can be effectively accomplished at any given time.
Such prior art system, while highly successful, nevertheless has several aspects which the present invention improves upon. First, when an RT sends a message, the message is most likely to be received by more than one base station. Each base station then transmits the message to the communications controller and thence to the host computer. The host computer thus receives duplicate messages, i.e., one message from each of the base stations receiving the transmitted message. When the network is extremely busy and operating at peak loads, the duplicate messages create a bottleneck and thereby restrict system throughput. Thus, one of the objects of the invention is to eliminate duplicate messages from being sent to the host computer. The general manner in which this object is accomplished is by operating the communications controller so that when duplicate messages are received by more than one base station, only one message is transmitted from the controller to the host. The one message is either an error free message received by a base station assigned to be the primary station for communicating with a specific RT, or in the case where the message to the assigned base station includes errors or is lost, an error free message from an unassigned base station.
Second, in the prior art system, when a sign-on message is received by a base station, the signal strength of the received transmission is measured by the base station. The base station then composes and sends to the communications controller a message which includes the signal strength and the ID of the RT sending the sign-on message. Since it is likely that a sign-on message will be received by more than one BS, duplicate sign-on messages are received by the CC. The CC then assigns the BS receiving the highest strength signal to the RT requesting a sign-on or attachment. The CC creates a table of assignments and sends such information to the host so that the host knows which BS is attached to which RT. Thus, another object of the invention is to avoid sending duplicate sign-on messages to a CC. This is accomplished by distributing the sign-on and attachment decision making process to the BSs and the RTs.
Third, making attachment decisions on the basis of signal strength does not permit "load leveling". Load leveling is a novel feature of the invention by which the attachment decision is made on the basis of not only the strengths of signals received by an RT from different BSs but also on the basis of the domain of each BS and the number of users currently attached to each BS. Thus, RTs are attached to BSs having limited range and fewer users thus allowing multiple BSs to use the one available operating frequency and simultaneously communicate with multiple RTs without interference.
Another object of the invention is to monitor the message error rate at each RT and request a re-evaluation, dependent on the error rate, of which BS the RT should be attached to.
Briefly, in accordance with the invention, a digital communication system has a plurality of RTs which communicate with a plurality of BSs by radio links operating at a single frequency. Each RT monitors the signal strengths of outbound messages and keeps an ordered table or list indicating which BSs produce the highest strength signals. At sign-on, an RT sends a message to the BS at the top of the list. The message includes the first predetermined number of entries at the top of the list. Each BS monitors the number of RTs it can hear and the number of users attached to each BS. The BS to which the sign-on message is sent, then decides on the basis of signal strength, number of users attached, and BS coverage, which BS should be attached to the particular RT and then notifies a communications controller.